Sheet metal articles can be made by hot stretch-forming processes that use complementary forming tools in a press under the pressure of a working gas to stretch-form a preheated sheet metal blank against forming surfaces on the forming tools. Such processes are particularly applicable to forming sheet metal into products of complex three-dimensional curvature such as automobile body panels.
In sheet metal hot blow forming processes, a hydraulic press is often used to support and move opposing forming tools that form a flat, pre-heated sheet metal blank into a three-dimensional contoured component. The hydraulic press is also preferred in order to provide balancing forces to oppose the high fluid pressures that build up between the forming tools. Hydraulic presses for shaping large parts typically open and close along a vertical axis. A vertically oriented hydraulic press, thus, has a lower platen for supporting one of the forming tools, often a punch or male finish-form tool, and an upper platen for carrying a complementary, opposing forming tool with a concave cavity, typically a female pre-form tool. The forming tools may be individually heated to maintain a suitable forming temperature for the sheet metal blank.
With hot blow forming processes, as well as with more conventional mechanical stamping processes, an engineer in charge of designing a manufacturing process will often choose a two-stage, two-station forming operation if the shape of the final body panel is impossible to form in a single stage. Such a two-stage process involves a first, pre-form stage and a second, finish-form stage that, together, minimize forming time and divide the total amount of material elongation to minimize the severity of panel deformation to yield a high-quality body panel. If, however, manufacturing economics dictate use of only a single station, then the engineer will often use double-action tooling. To operate such double-action tooling, the press can be a double-action press, wherein a secondary action of the press is generally used to clamp a sheet blank and perform first-stage forming.
In double-action hot stretch-forming, the sheet metal blank is inserted between the forming tools while in their open position and the press moves the forming tools from the open position to a first stage pre-forming position. Here, the edges of the blank are gripped between opposed binder surfaces of the opposed forming tools and gas pressure is applied to one side of the blank so that a central part of the blank is stretched against a pre-forming surface of the pre-form tool. Then, the opposing finish-form tool is moved closer to the now pre-formed blank in a second stage forming position. Gas pressure is now applied to an opposite side of the blank so that the central part of the blank is stretched against a finish-forming surface of the finish-form tool to complete the shaping of the blank. The press then opens for removal of the formed component and insertion of a new blank.
Hot stretch-forming of certain automobile body panels, however, poses unique challenges for tooling designers. For example, a deck lid, which, when mounted to an automobile, has a generally horizontal surface for covering a top of an automobile trunk and has a generally vertical surface for defining a rear end of the trunk. Both surfaces usually have complex curved shapes or features, such as at the corners thereof or deep pockets in the vertical surface for license plate or stop lamp recesses. One challenge is that such complex features are not amenable to the traditional vertical motion of the press and forming tools, because the complex features are disposed at a negative draft angle with respect to the vertical axis of the press motion. Accordingly, there exists a die lock condition between the finish-form tool and the finish-form blank. In other words, after second stage forming, the finish-form tool cannot be vertically retracted away from the finish-form blank, or vice-versa, without binding or interference between the complex features on the finish-form tool and the complex features of the pre-form blank.
Thus, there is a need for hot stretch-forming tooling designs that better accommodate part features disposed at a negative draft angle to avoid die-lock conditions.